The stable propagation of our genomes through cell division depends on the proper organization of complex and dynamic microtubule-based structures, such as the bipolar spindle prior to anaphase and the central spindle (or midzone) during anaphase. Our long-term goal is to determine the molecular mechanisms required for the assembly and function of these cytoskeletal structures. To achieve this goal we take an interdisciplinary approach in which we combine structural, biophysical, chemical and cell biological methods. In the current proposal, which builds upon recent publications and unpublished preliminary data, we focus on the following three Aims: (1) Gain structural insights into antiparallel microtubule crosslinking during cell division. X-ray crystallography, electron microscopy, and TIRF (total internal reflection fluorescence) microscopy assays will be used to examine how PRC1, a widely conserved non-motor microtubule associated protein (MAP) required for central spindle assembly, selectively crosslinks antiparallel microtubules. We will also analyze how PRC1 binds kinesin-4 to control antiparallel microtubule overlap length. (2) Examine how microtubule organization and function are regulated in dividing cells. Structure-guided mutagenesis, in vitro assays, and high-resolution microscopy will be combined with chemical inhibitor and RNAi/add-back based perturbations to examine how central spindle assembly and function are regulated at two different levels: (a) Nanometer-scale features, such as antiparallel microtubule overlap, recognized by PRC1, and (b) Micron-scale spatial activity patterns, such as a spatial phosphorylation gradient that depends on Aurora kinase, a widely conserved regulator of several key processes required for successful cell division. (3) Analyze the mechanical properties of microtubule-based structures essential for cell division. We will use a dual force-calibrated microneedle-based system to examine the viscoelastic properties of the metaphase spindle. Chemical and biochemical perturbations will be used to link the known biochemical and biophysical properties of key proteins to the spindle's micro-mechanics. Together, our findings should advance our understanding of key proteins and regulatory cues (biochemical or mechanical) that contribute to the self-organized assembly of microtubule structures required for the stable propagation of our genomes. Whole chromosome loss, which can arise due to errors in cell division, has been linked to developmental defects and diseases in humans. Our research should provide insight into the molecular mechanisms that ensure cell division is completed without error. Our studies should also contribute to finding new targets for therapeutic agents and impact the development of drugs that target proteins needed for cell division.